How viruses have shaped our DNA

Twenty-five thousand years ago, the first elements of modern humanity were beginning to emerge in Europe. One of the earliest known settlements, near the Dyje river in what is now the Czech Republic, was founded; a place where people lit fires, spun thread, wore clothes and caught fish on hooks. Ceramics had just been developed, and people began to form clay or to carve other materials into likenesses of men, women and deities. Humans had reached an important new stage on the journey that had begun two million years earlier with the sharpening of the first flint knife.

Then, in large numbers, they began to fall ill.

How can we say that a virus caused a pandemic, 20,000 years before the beginning of recorded history? David Enard, an evolutionary biologist who works at the University of Arizona, says the scars of ancient epidemics, and the history of our long and ceaseless wars with viruses, are written in our DNA. Finding out what caused them may help us to predict the next pandemic.

Around 30 per cent all the adaptive mutations in our genome – a third of human evolution – has been in response to viruses.

Enard’s work involves searching human genomes for evidence of ancient epidemics. It has led him to believe the current outbreak is one in a series of hundreds or even thousands of epidemics and pandemics that have afflicted humans through every moment of our evolution. In 2016, while he was at Stanford University, Enard showed that around 30 per cent all the adaptive mutations in our genome – a third of human evolution – has been in response to viruses.

Even within the span – vanishingly brief, in evolutionary terms – of recorded history there are examples such as the Black Death, caused by the bacterium Yersinia pestis, the devastating influenza pandemics of 1889 and 1918, and the plagues that ravaged the Greeks, Romans, Aztecs and many others. “But if you start thinking in terms of hundreds of thousands of years of evolution … it leaves a lot of time for hundreds or even thousands of pandemics to have occurred.”

Enard finds the evidence of these ancient plagues by looking at the effect they had on our evolution. Early people would have had some knowledge of medicine, but this would have done nothing to prevent or cure a new infectious disease. The only defence they had was that which they carried in their genes.

“Mutations are usually harmful,” Enard explains, “but when a pandemic happens, it could be that some individuals have mutations that actually make them resistant against the infection.”

There are several ways that genes could confer an advantage. One kind of mutation, Enard says, could be a change in the immune system, which would make the immune cells of these people more able to control the virus early on, so that the virus does not multiply so much.